1. Field of the Invention
This invention relates to novel liquid crystal compounds having a positive dielectric anistropy, and liquid crystal compositions containing the same.
2. Description of the Prior Art
Liquid crystal display elements utilize optical anisotropy and dielectric anisotropy of liquid crystal substances, and are classified into various types such as TN (twisted nematic) type, DS (dynamic scattering) type, guest-host type, DAP type, White-Tailor type, etc. according to their display modes, and the properties of liquid crystal substances required for their respective uses are different. For example, liquid crystal substances having a positive dielectric anisotropy .DELTA..epsilon. are required or those having a negative one are required or those having an intermediate value therebetween are suitable, depending on the kinds of display elements. Anyhow, however, in any mode, liquid crystal substances to be used are necessary to exhibit a liquid crystal phase in temperature ranges as broad as possible, around room temperature, and also to be stable to heat, air, light, etc. At present, however, no single compound which alone satisfies all such conditions is present, and it is the present status that several kinds of liquid crystal compounds or these compounds and non-crystalline compounds are mixed to obtain those which withstand practical uses for the present.
Recently, need particularly for liquid crystal display elements which can be driven at low voltages has increased, and in order to satisfy such a requirement, liquid crystal compositions having a larger .DELTA..epsilon. value are usually required.
In general, liquid crystal compositions having an optional .DELTA..epsilon. value can be obtaind by adequately mixing compounds having a positive .DELTA..epsilon. value with those having a negative .DELTA..epsilon. value. Thus, in order to obtain a liquid crystal composition having a large positive .DELTA..epsilon. value, a component having as large a positive .DELTA..epsilon. value as possible will be to be used. In this case, however, there should be used those which have a good compatibility with other components and extend the mesomorphic range of the resulting composition or at least do not narrow it. In order to satisfy such an object, the present inventors previously invented carboxylic acid 3-chloro-4-cyanophenyl esters expressed by the general formula ##STR2## wherein R represents an alkyl group or alkoxy group of 1 to 10 carbon atoms and ##STR3## represents benzene ring or cyclohexane ring,
and applied for patent (Japanese patent application laid-open No. Sho57-154158/1982). Recently, however, liquid crystal display elements having a higher level of performances have been needed, and compounds having a lower viscosity and a good stability to heat, light, air and moisture have been required. The present invention has been made to meet such a demand.